1. FIELD OF THE INVENTION
This invention relates to improvements in amplifier circuits, and, more particularly, to improvements in differential amplifiers that result in high power supply rejection ratios.
2. BACKGROUND INFORMATION
It is often desired to be able to use an amplifier having a differential output in applications in which a high power supply rejection ratio (PSRR) is needed, particularly in operational amplifiers and the like. However, unless extremely precise layout precautions and matching of component parts is employed, achieving a high power supply rejection ratio is difficult. This is particularly true in circuits that use low voltage power supplies, an increasing trend in portable device technology.
In the past, various solutions have been advanced toward solving this problem. One way that has been used is to provide multiple cascode stages. However, particularly in the low supply voltage applications mentioned, cascoding of stages is impractical.
The problem is further exacerbated by limitations on the operating range of the circuitry. Increasingly popular circuits are so-called "rail-to-rail" circuits that provide dc operating ranges to voltages as close as possible to the voltages of the power supplies.
In such circuits, fully differential operation necessitates sensing of the common mode input voltage through current mirrors used in combination with various current sources. Aside from the problems of balancing the current values provided from the various current sources, mirror circuits then receive cascoding of their inputs and outputs, and are difficult to use in circuit embodiments in which differential outputs are to be provided under low-supply constraints.